A water absorbent resin (alternately referred to as “water absorbent polymer, SAP/Super Absorbent Polymer”) is a water-swellable, water-insoluble polymer gelling agent, and has been frequently used primarily in disposable applications as water absorbent articles such as paper diapers and sanitary napkins, as well as water retention agents for agricultural and horticultural use, industrial water stopping materials, and the like. As such a water absorbent resin, many monomers and hydrophilic polymers have been proposed as a raw material. Among others, a polyacrylic acid (salt)-type water absorbent resin which uses acrylic acid and/or salts thereof as a monomer has been industrially most frequently used due to its high water absorption performance.
Such a water absorbent resin is obtained by polymerizing an aqueous monomer solution to obtain a water-containing gel-like crosslinked polymer, subjecting the water-containing gel-like crosslinked polymer to fine granulation during or after the polymerization, drying the resultant particulate water-containing gel-like crosslinked polymer, and subjecting the dried polymer to steps such as pulverization, classification, and surface crosslinking. As an apparatus for drying the particulate water-containing gel-like crosslinked polymer, a through-circulation band dryer or the like can be used (Non-Patent Literature 1).
For drying conditions of a particulate water-containing gel-like crosslinked polymer, various methods have been proposed for enhancement of physical properties of a water absorbent resin to be obtained (for example, reduction in residual monomers, enhancement of absorption capacity, and reduction in extractables, and the like). Specifically, for the purpose of reducing residual monomers, a method of controlling a dew point or temperature in an atmosphere inside a dryer (Patent Literature 1) has been proposed, and for the purpose of repressing changes in physical properties as a result of drying, a method of using, at the time of drying a water-containing gel-like crosslinked polymer with a through-circulation band dryer, an upward warm air for former half part in drying step and a downward warm air for latter half part in the drying step, and drying the polymer with a gas at a predetermined rate (Patent Literature 2) has been proposed.
In a method of drying a water-containing gel-like crosslinked polymer with a band dryer, a method of measuring a difference between a pressure on a side where hot air is blown to a hydrogel polymer layer and a pressure on the opposite side thereof, in order not to produce any undried product (Patent Literature 3), and a method for drying hydrogel polymer, which for the purpose of uniform drying, comprises setting a roll having plural pins protruded on the peripheral surface and having a rotation axis perpendicular to a conveyance direction of a conveyor on the conveyor at conveyance initiation point, starting operation of the conveyor, placing a hydrogel polymer in front of the roll and passing the polymer between the roll and the conveyor at the initiation of conveyance while the roll is rotated, prior to drying the polymer (Patent Literature 4) have been known. A drying method which comprises, in drying a hydrogel polymer, drying the hydrogel polymer until a moisture content thereof reaches within a specific range, maintaining the same conditions for 10 minutes or more, and then carrying out finish drying (Patent Literature 5) has been also known. Further, a drying method which comprises controlling drive of a continuous belt type dryer by continuously measuring a moisture content of a hydrogel polymer during or after drying (Patent Literature 6) has been known.